The present invention relates generally to an image scanner with improved image processing speed, and more specifically to an image scanner in which a double-side read mode for reading image data on the right and reverse sides of a document with improved resolution through the optimum signal processing is provided to read image data on the right and reverse sides of a double-sided manuscript simultaneously and the read image data is transmitted to a host computer at high speed; the image data obtained being subjected to white level control.
Image scanners used for entering data in electronic filing systems are known as one of image input units for entering image data into computers. In banks, insurance companies and other financial institutions, for example, various items written on an enormous amount of business documents, such as contract documents produced in the course of business transactions, are entered into a computer. In order to perform input processing with accuracy and high speed and implement the retrieval of data, an electronic filing system is built, into which the contents of business documents are entered into a computer in the form of image data via electronic filing units as key components of the electronic filing system. Image scanners are therefore essential input units for entering the contents of business documents in the form of image data.
An image scanner acquires image data in the following manner: Assume that a plurality of business documents are placed by the operator on a document feeder. The image scanner feeds a sheet of business documents from the document feeder to a reader. The line sensor (CCD) of the reader reads image data, such as characters on the document, which are reduced by a data processor to a form that can be processed by a computer (that is, digital data, or binary image data). The data are then transferred to a host computer. Upon completion of reading, the document is discharged to a document discharge tray.
In general, image scanners are required to have high image quality for the image data read by them. Among a large number of competing products in the market, image scanners having a higher resolution in the same price range enjoy a greater demand. Resolution is basically determined by the reading accuracy of the optical system in the read means of the scanner. To improve the resolution of an image scanner, it is necessary to use the optical system of the read means with a higher reading accuracy.
The use of an optical system having too high a resolution, however, is not a practical approach because it tends to increase the cost of the image scanner. It is therefore desirable to increase the resolution of an optical system without changing the configuration of the optical system of the read means To this end, it is necessary to improve resolution indirectly by performing signal processing to improve resolution, instead of directly improving resolution through the improvement of the optical system as hardware.
Image scanners are also required to have an improved speed of reading image data. As in the case of resolution, an image scanner having higher read speed enjoys much demand. When improving resolution through signal processing, therefore, the signal processing must not impede the efforts to increase read rate. With too complex signal processing, real-time signal processing becomes impossible with increases in the amount of image data. Thus, it is necessary to implement the optimum signal processing for improving resolution.
Aside from the above discussion, there is a demand for entering contract provisions and other items on both the right and reverse sides of a contract document from considerations of paper saving and other social requests. Furthermore, it has been widely practiced to build an electronic filing system making use of its superior functions. Under these circumstances, image scanners are increasingly required to have a higher speed at which data are entered to handle the increasing amount of data to be entered into computer using image scanners.
Conventional image scanners, however, are mostly of a type that can read image data on only one side of a document, that is, a one-side scanning type. The use of a conventional image scanner to enter into a computer the image data in a document on both sides of which characters and other data (image data) are contained (double-sided document) involves two reading operations. Upon completion of the image scanner reading the image data on a right side of a document, the operator has to turn over the document discharged on the document feeder to cause the scanner to read the image data on the reverse side of the document. With this method, the reading of a double-sided document involves the intervention of the operator to turn over the document to cause the scanner to read the reverse side, requiring additional time for two reading operations and operator's paper handling. This imposes an obstacle in achieving high-speed image data entry.
Interfaces for image scanners in most of existing computers (or electronic filing systems) have been established as ones compatible with conventional image scanners (of the one-side scanner type). It is not desirable to change this established interface format. If a new interface for transferring the image data contained in a double-sided document is developed and adopted, the interface for double-sided scanning has to be added to the existing computers, posing a heavy burden to users. Consequently, image scanners capable of outputting the image data contained in a double-sided document are required to have connectivity to computers without changing the operating environment of the conventional one-side image scanners. In other words, it is necessary to increase the speed of double-side image scanners, while maintaining interchangeability with conventional image-data interfaces in both logical and physical terms.
The rate of image-data input cannot be improved if the image data read from a double-sided document are transferred to the computer at an existing data transfer rate. Transferring the amount of image data increased twice as much per unit time by reading both sides of a document at the existing data transfer rate could disturb the continuous supply of paper. It should be noted that the rate of image-data input processing is dependent on data transfer rate to the computer, rather than on the reading capacity (drive frequency) of the line sensor (CCD). This means that even when both sides of a document are read simultaneously, the computer throughput of documents per unit time remains almost the same (about 1/2 sheets). This also makes it necessary to increase data transfer rate.
Aside from the above discussion, there can be other means to increase the speed of image-data input. That is, the image data of a double-sided document can be obtained by reading the image data on the right and reverse sides of a document simultaneously with separately provided image-data read sensors each dedicated to read the right or reverse side. This is tantamount to virtually doubling image-data reading rate. Thus, the speed of image-data input processing can be increased by increasing (doubling) signal processing rate to form image data.
Consideration should be given here to white level control. When image data is formed by subjecting the read image data to a predetermined signal processing, image data must be read clearly regardless of the types of paper, whether wood-free paper or newsprint. It is necessary therefore to obtain improve contrast in the image, allowing for varied shades of document paper, and compensate for the uneven density of the manuscript, or changes in the amount of light of a light source. For this reason, white level control is carried out repeatedly during the period when the image data of a document is being read. A microprocessor for executing the control of the image scanner executes a white level control program at predetermined intervals. The execution of a white level control program is an interrupt processing to which high priority is given (see FIG. 20B). The white level control program changes the white level value from time to time in accordance with the degree of shades of paper, the uneven density of the manuscript, and changes in the amount of light of a light source. By doing so, image data on almost all types of paper can be read clearly.
The results of a study conducted by the present inventor revealed that it is almost impossible in practice to process the image data of a double-sided document that is twice as much as the amount of data on a single-sided document with conventional signal processing, particularly with white level control of the conventional type. Even when image data on the right and reverse sides of a document are read, means for reading image data clearly in accordance with changes in the shades of paper are essential. In this respect, white level control cannot be omitted. White level control, however, may place a burden on the microprocessor because the simultaneous reading of the right and reverse sides of a document inevitably doubles the amount of data being processed. In practice, the fact that white level control is performed as a high-priority interrupt processing may preclude the microprocessor from carrying out other processing for controlling the image scanner.
A (higher-speed) microprocessor having excellent signal processing performance can be used for controlling the image scanner. With this arrangement, the above problem can be solved, but the cost of the image scanner may increase only to realize white level control. This makes this approach impractical.